fix: specify lowest node version in package.json

The lowest version of node required by explicit dependencies is ^8.12.0 specified by Execa

index.js

@@ -6,7 +6,7 @@ const pkg = require('./package.json')
 require('please-upgrade-node')(
   Object.assign({}, pkg, {
     engines: {
-      node: '>=8.6.0'
+      node: '>=8.12.0'
     }
   })
 )